FGF-23 is a member of the fibroblast growth factor (FGF) family and a polypeptide containing of 251 amino acids, which is produced mainly in bone tissues and acts on the kidney to inhibit reabsorption of phosphorus in the renal tubules. In recent years, FGF-23 has been suggested to be involved in diseases such as hypophosphatemic rickets, tumor-induced osteomalacia, and renal failure (see Non-Patent Document 1).
FGF-23 is formed by dissociation of the 24 amino acids at the N-terminus to give a polypeptide containing 227 amino acids, and modified to add sugar chains, and released to the outside of cells as an approximately 32.5-kDa mature protein. Moreover, the bond between position 197 and position 198 from the N-terminus FGF-23 is cleaved by thrombin, and the fragment of position 198 to position 251 exists in the blood as a C-terminal fragment.
To date, antibodies against FGF-23 have been Obtained (see Patent Documents 1 and 2; and Non-Patent Document 2), immunoassays for FGF-23 in the serum or plasma which use these antibodies have been reported (see Patent Document it and Non-Patent Document 2), and measurement kits based on these measurement methods are also commercially available [Human Intact FGF-23 ELISA Kit (Immutopies), Human FGF-23 (C-Term) ELISA Kit (Immutopies), and FGF-23 measurement reagent (Kainos)].
However, immunoassays to date are measuring methods using plates, and these methods have the problems of having low measurement sensitivity and a narrow measurement range. Particularly in chronic kidney disease (CKD) patients and dialysis patients, the specimens are M the concentration range of several pg/mL to several tens of thousands of pg/mL, and there were problems such as, in low concentration samples, correct measurement values could not be obtained due to poor accuracy and, in high concentration specimens, the measurement range was surpassed.